imap_open() : which TLS version does it use?

i need to connect to a dovecot server running on ubuntu 20.04, from PHP
7.4.3 running on the same server.

i've verified that dovecot is listening on port 143.

i've set

ssl_min_protocol = TLSv1.2

in /etc/dovecot/conf.d/10-ssl.conf

but in my web app, i now get :

{192.168.178.21:143/imap/ssl}: "FAIL:[\n \"TLS\\/SSL failure for
192.168.178.21: SSL negotiation failed\"\n]"

so i'm thinking the TLS protocol version that imap_open() in PHP is off
target.

how do i correct this?